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I've worked on a bunch of SU carbs on Jag's, MG's, Triumph cars, Aston Martins and more that I can't remember now.
That fitting as you suspect is most likely for a vacuum advance distributor on a car.
It should be capped if not connected to a distributor. If left open, It will interfere with proper response of the variable venturi (piston) and may contribute to a weak mixture at idle or a lean flat spot just off of idle..